In conventional alternating current power systems, power is transmitted from an AC power source, through a network of power lines and through transformers to supply either three-phase or single-phase loads. These loads typically consist of both linear and non-linear elements. Power is provided on the power system at a fundamental frequency, f.sub.0, typically 60 Hz. However, certain non-linear loads also draw undesired currents having frequencies Other than the fundamental line frequency (e.g., harmonics f.sub.2 =2f.sub.0, f.sub.3 =3f.sub.0, etc.). These undesired current components are referred to as non-fundamental frequency currents.
Non-linear loads, such as AC-DC rectifier power supplies, draw non-fundamental frequency currents. Typically, these non-fundamental frequency currents are supplied from the AC power source to the load along power lines and through transformers. Excessive non-fundamental frequency currents can produce serious voltage distortion on power lines when they interact with the source impedances of the power system (including the impedances of the AC power source and the power lines). Excessive non-fundamental frequency currents and their associated voltages can also cause interference, overheating of power lines and transformers and malfunctioning of other equipment connected to the upstream AC power system.
An AIM filter is an electronic power conversion device that is connected in shunt across a power line to compensate for non-fundamental frequency currents drawn by a load. The AIM filter is typically located near the load which draws the undesirable non-fundamental frequency currents. In general, an AIM filter senses the non-fundamental frequency currents being drawn by the load, attempts to generate currents which match these non-fundamental frequency currents, and injects these matching non-fundamental frequency currents back onto the power line. In this manner, the AIM filter provides the non-fundamental frequency current components required by the load so that these currents do not flow from the AC power source to the point where the AIM filter connects to the power line.
An AIM device can also be used to test the response of an AC power source to nonlinear loading or the imperviousness of a load (e.g., electronic equipment) to poor quality electrical power. In such an application, the AIM device does not sense the current present on the power line, but instead generates predetermined non-fundamental frequency current components. These current components are then injected onto a power line that might not otherwise experience non-fundamental frequency currents. The response of the AC power source or the load is then observed to determine how it will perform when exposed to such conditions.
FIG. 1 is a single-line diagram of a prior art AIM filter 17 connected to an AC power system. AC power source 10 transmits a current of I.sub.S at a voltage of V.sub.LINE to load 12 on power line 11. Load 12 generally includes a linear element 41 having impedance Z.sub.L and a non-fundamental frequency current generator 42, which draw currents I.sub.ZL and I.sub.H, respectively.
Current transducer 15 is coupled to power line 11 to sense the current IL drawn by load 12. Current transducer 15 provides a signal V.sub.10, which is proportional to current I.sub.L, to harmonic current detector 16. Harmonic current detector 16 filters the fundamental frequency current component of signal V.sub.10 to produce a voltage signal V.sub.11 proportional to the non-fundamental frequency current components being drawn by load 12, i.e. the non-fundamental frequency current components of both I.sub.ZL and I.sub.H.
Voltage signal V.sub.11 is transmitted through summing node 18, amplifier 19 and summing node 20 of transconductance amplifier 24. In response, transconductance amplifier 24 generates a current output I.sub.1 which is proportional to input voltage signal V.sub.11. The gain of transconductance amplifier 24 is controlled so that the current I.sub.1 produced by transconductance amplifier 24 is equal to the non-fundamental frequency current components of the load current I.sub.L.
Transconductance amplifier 24 typically includes a pulse width modulator circuit 23, a high-speed switching circuit 26, an output inductor 30 and a current transducer 28. Transducer 28 provides a current feedback signal V.sub.12 to summing node 18 to make the amplifier a transconductance amplifier, i.e. an amplifier that produces an output current in response to an input voltage signal. Inductor 30 and passive filter 32 remove high frequency switching current components introduced by pulse width modulator circuit 23 and transistor switching circuit 26.
Voltage sensor 22 provides a line voltage feed forward signal inside the current control feedback loop that reduces the necessary gain of the current control feedback loop, increasing the stability of that loop.
The output current I.sub.1 of transconductance amplifier 24 produces all of the non-fundamental frequency current components of load current I.sub.L, including I.sub.H and the non-fundamental frequency components of I.sub.ZL. Thus, the effect of current I.sub.L, is to isolate the non-fundamental frequency components of linear element 41 from the AC power line and unload any non-fundamental frequency voltage components of V.sub.LINE. This unloading can result in instability of the power system and increased non-fundamental frequency voltages on the power system.
For transconductance amplifier 24 to produce the desired flow of current on power line 11, the transconductance amplifier 24 must be able to generate an instantaneous voltage V1 equal to EQU V.sub.LINE (t)+L*dI1/dt
where V.sub.LINE (t) is the AC power line voltage, V.sub.LINE, at time t at node 13, L is the inductance of inductor 30 and dI1/dt is the derivative of output current I1 with respect to time.
In AIM filter applications such as the one described above, dI1/dt can be substantial, especially in three-phase systems. For a typical non-linear rectifier type load, the waveform of load current I.sub.L comprises alternating positive and negative rectangular current pulses with fast rising and falling edges, i.e. a large dI1/dt. The voltage that must be produced across inductor 30 (L*dI1/dt) to produce the fast rising and falling edges of the current pulses can approach the peak value of the line voltage V.sub.LINE. The peak value of dI1/dt typically occurs when the line voltage V.sub.LINE is between 50% and 100% of its peak value. Consequently, the required voltage V1 which must be generated at the output of switching circuit 26 approaches twice the peak voltage V.sub.LINE of power line 11. If the transistor switching circuit 26 is not capable of providing the required voltage V1, transconductance amplifier 24 will not be able to produce the desired output current I1.
Even at the lowest typical voltages of V.sub.LINE (e.g. 208 Volts AC line-to-line), the peak output voltage V1 of switching circuit 26 must be at least 600 V to provide the desired output currents. The fast switching devices needed to generate high frequency currents at voltages above approximately 600 V are either not available or expensive. Furthermore, the switching losses in the switching circuit 26 increase substantially as the output voltage V1 increases, thereby resulting in a loss of efficiency within the transconductance amplifier 24.
Guidelines presently being discussed for harmonic suppression of utility line connected equipment and adopted as specifications for some new installations place lower limits on the allowable high frequency harmonic currents than on the allowable lower frequency harmonic currents. (See, e.g., IEEE Practices and Requirements for Harmonic Control in Electric Power Systems, IEEE Standard 519-1992.)
It would therefore be desirable to have an AIM filter having a switching stage capable of operating at a reduced voltage.